When a cylindrical magnet having a lot of magnetic poles on a peripheral surface rotates around a cylindrical shaft, magnetic flux densities of the cylindrical magnet in radial and circumferential directions at a position separate from the peripheral surface by a predetermined distance (r1) change substantially sinusoidally with the rotation angle. In this case, the amplitude of magnetic flux density in a radial direction is as large as about 1-2 times that of magnetic flux density in a rotational direction.
A rotation-angle-detecting apparatus can be constituted by a sensor device (magnetic sensor comprising pluralities of spin-valve, giant-magnetoresistive devices) disposed at a position separate from the magnet by the distance (r1), to measure magnetic flux densities in radial and rotational directions.
Because the sensor device detects not a magnetic field intensity but the direction of a magnetic flux, magnetic flux densities with different amplitudes in radial and rotational directions cause a sensor device having a magnetosensitive axis in a radial direction to generate a first output voltage (Vx) in a trapezoidal waveform, and a sensor device having a magnetosensitive axis in a rotational direction to generate a second output voltage (Vy) in a triangular waveform. Using a sensor device having two perpendicular magnetosensitive axes on one substrate, the first and second output voltages can be obtained simultaneously.
When a magnetic flux density is within a certain range, the amplitudes of output voltages are substantially constant regardless of the maximum magnetic flux densities in radial and rotational directions. Namely, the first and second output voltages are Vx=cos θmag and Vy=sin θmag at a magnet rotation angle θmag as described later, with their amplitudes substantially equal, though actual waveforms including harmonics are trapezoidal and triangular. Thus, the first and second output voltages have different waveforms, and their amplitudes are substantially equal despite different magnetic flux density amplitudes, resulting in measured rotation angle errors (angle errors).
JP 2006-023179 A discloses a magnetic position detection apparatus comprising a magnetic member, and plural pairs of vector-detection-type magnetoresistive devices opposing magnetic poles of the magnetic member, plural pairs of the vector-detection-type magnetoresistive devices being arranged such that their magnetosensitive planes are substantially parallel to an external magnetic field generated from the magnetic member, and that the magnetization directions of pin layers in a pair of the vector-detection-type magnetoresistive devices are deviated from each other by substantially 90°, and all of the vector-detection-type magnetoresistive devices being disposed at the same position in the magnetic pole arrangement direction of the magnetic member. JP 2006-023179 A describes that the relative movement of the magnetic member and the vector-detection-type magnetoresistive devices shown in FIG. 1(A) provides two sinusoidal outputs with 90° phase deviation.
As described in JP 2006-023179 A, two sinusoidal outputs with 90° phase deviation are obtained only when an amplitude ratio of a magnetic flux density in the movement direction to a magnetic flux density in a direction vertical to the magnetic member is substantially 1 (when the magnetic poles have extremely large transverse sizes relative to the magnetization pitch of the magnetic member). However, because the magnetic flux density amplitudes are different, output voltages supplied from magnetic field detectors actually have trapezoidal and triangular waveforms having high-order harmonics, resulting in position detection errors, thus failing to achieve accurate measurement.
JP 2006-194861 A discloses a method for detecting a rotation angle using magnetoresistive devices, in which signal phase errors, deformation errors, etc., in outputs from the magnetoresistive devices are reduced by various waveform adjustments. However, it fails to describe a method for reducing errors due to unevenness (amplitude differences of an external magnetic field, etc.) inevitably occurring in a rotating parallel magnetic field.
JP 2006-023179 A and JP 2006-194861 A never describe nor suggest that because a magnetic flux density in a radial direction has a larger amplitude than that of a magnetic flux density in a rotational direction, output voltages have different waveforms in radial and rotational directions, resulting in angle errors. In the technologies described in these references, the transverse size of magnetic poles should be, for instance, about 100 times the magnetic pole pitch, to make the magnetic flux density amplitude in the movement direction equal to that in a direction vertical to the magnetic member. However, it is not practical to use such a large magnetic member for the detection of a rotation angle or a moving distance.
As described above, when two output voltages obtained from sensor devices capable of detecting the direction of a magnetic flux are subjected to arctangent calculation to determine the electric or rotation angle of a magnet rotor, a rotation-angle-detecting apparatus provides large angle errors.